Signal transmission system

ABSTRACT

A single wire interface for a transducer transmits the transducer output as a frequency modulated signal over one single wire during one interval. During a second interval a reference signal is transmitted as a frequency modulated signal. Both the transducer output and the reference signal output are processed by the same circuitry during the respective intervals to provide both frequency modulated signals. The frequencies of the two signals are measured and then the ratio of the two periods which is the reciprocal of the two frequencies is calculated. This ratio is the direct measure of the output of the transducer and when provided eliminates sources of errors.

FIELD OF THE INVENTION

This invention relates to a signal transmission system for sensors andmore particularly to a signal transmission and measuring system forsensors employing a single transmission wire and a grounded return.

BACKGROUND OF THE INVENTION

As one can ascertain, the prior art is replete with pressure transducersor sensors employed in harsh environments. Such environments includedeleterious substances which may destroy the transducer, as well as highpressures and temperatures. High temperatures include those temperatureswhich are found in various high temperature environments as combustionengines, for example. In other applications, such as the use of pressuretransducers in injection molding and for other environments extremelyhigh temperatures are also found. The prior art has disclosed pressuretransducers which are capable of operating at very high temperatures astemperatures in excess of 600° C. See for example, U.S. Pat. No.7,124,639 which issued on Oct. 24, 2006, entitled “Ultra HighTemperature Hermetically Protected Wirebonded Piezoresistive Transducer”by A. D. Kurtz et al. and assigned to Kulite Semiconductor Products,Inc., the assignee herein. See also U.S. Pat. No. 6,363,792 entitled“Ultra High Temperature Transducer Structure” issued on Apr. 2, 2002 toA. D. Kurtz et al. and assigned to the assignee herein. See also U.S.Pat. No. 6,530,282 entitled “Ultra High Temperature TransducerStructure” issued on Mar. 11, 2003 to A. D. Kurtz et al. and assigned toKulite Semiconductor Products, Inc., the assignee herein.

By referring to the above noted patents, one can see applications ofsuch transducers in high temperature environments as well as themonitoring of such signals in such environments. One problem is foundwhen one deals in the oil and geothermal exploration fields. In such oiland geothermal explorations, one uses pressure or temperaturetransducers which are exposed to temperatures much higher than thoseexperienced by standard electronics. Typical transducers which are usedfor normal operations are usually limited to temperatures below +125° C.Due to the depth of drilling as well as the use of steam to extract theoil the operating temperature in such explorations exceed 200° C.Pressure transducers using a piezoresistive silicon-on-insulator (SOI)structure are widely used in such applications. Such transducers forexample are described in the above noted patents. Also used are platinumresistors (RTD) used to measure the temperature which resistors are alsocapable of operating at these high temperatures. Thus, the combinationafforded in regard to the above is that one requires a pressuretransducer which can operate at high temperatures and one also requireselectronics which can operate at such temperatures. See for example aco-pending application entitled “High Temperature Pressure SensingSystem” Ser. No. 11/234,724 filed on Sep. 23, 2005 for A. D. Kurtz etal. and is assigned to the assignee herein. In that application, thereis described a high temperature pressure sensing system which includes atransducer having pressure sensing piezoresistive elements formed by aSOI process. The system also uses SOI CMOS electronic circuitry which isoperatively coupled to the piezoresistive sensor as well as ancillarycircuitry connected to the unit to provide compensation andnormalization. That application is incorporated by reference in itsentirety herein.

Other examples of SOI technology may be seen in U.S. Pat. No. 5,955,771entitled “Sensor for Use in High Vibrational Applications and Methods ofFabricating the Same” issued to A. D. Kurtz and U.S. Pat. No. 4,672,354.

In existing oil and geothermal applications, due to the depth of thedrilling as well as due to the use of steam to extract the oil, veryhigh temperatures are involved. In oil and geothermal explorations thewires used in these systems are extremely long and can be as long as10,000 meters. These wires apart from being extremely long are alsoexpensive. The cost of the wire often exceeds the cost of thetransducers. In prior art applications, the pressure transducers areconnected to the wiring via a four-to-twenty milliamp electronicinterface. The second wire is the metal conduit in which the wire isinserted. The prior art method has significant temperature limitationswhich are further aggravated by the significant power dissipation of thefour-to-twenty milliamp interface. This power dissipation increases thejunction temperature of the electronics by several tens of degrees aboveambient temperature. The prior art method also requires a separate wirefor each pressure or temperature sensor.

The present invention discloses a way of interfacing one or morepressure sensors to a measuring system using only one wire for thesignal and power and a return wire which is usually the conduit of thesignal/power wire. An electronic interface is advantageous for sensorslocated in a very high temperature environment at great distances fromthe measuring system such as described above in the oil and geothermalexplorations. The invention is also well suited for integration in acircuit using technology suitable for high temperature operation as thesilicon-on-insulator (SOI) process. The signal transmission system orwire interface described is also applicable and advantageous for use insystems operating at more benign temperatures and over shorter distancesas it simplifies the wiring as well as the measuring method.

SUMMARY OF THE INVENTION

Apparatus for transmitting a transducer signal to be measured from asignal generation location to a measuring location connected by a singlewire where undesirably the transducer signal is subjected to variationscaused by multiple sources. The apparatus comprises a transducerpositioned at the signal generation location and operative when biasedby a power source to provide an output signal according to a monitoredcondition. A reference level generator is coupled to the power sourceand operative to provide a reference level output proportional to thevalue of the power source. A multiplexer for receiving at one multiplexinput the transducer output signal and at another input the referencelevel output to provide at a multiplexer output the transducer signalfor a first interval and the reference level for a second interval. Aconverter responsive to the multiplexer output for converting thetransducer signal to a first frequency modulated signal having afrequency output variation according to the value of the transduceroutput signal during the first interval and for providing a secondfrequency modulated signal indicative of the reference level during thesecond interval, where any variations in signals which may be caused bymultiple sources are present in both signals; and measuring arrangementpositioned at the measuring location and responsive to the modulatedsignals to provide the ratio of the periods of the signals, where theratio is a direct measure of the transducer output signal with theundesired variations substantially eliminated.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of a one wire system directed from a signalgeneration located to a measuring location according to an embodiment ofthe invention.

FIG. 2 is a block diagram of a wire interface located at the signalgeneration location according to an embodiment of the invention.

FIG. 3 consists of FIGS. 3A and 3B and depicts timing diagrams asshowing the pressure and reference intervals according to an embodimentof the invention.

FIG. 4 consists of timing diagrams showing the output of a capacitor anda monostable multivibrator operating according to an embodiment of theinvention.

FIG. 5 is a block diagram of a signal measuring arrangement according toan embodiment of the invention.

FIG. 6 is an alternate embodiment of a signal measuring system accordingto an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 there is shown a block diagram of a one wire systemfor measuring a transducer output according to this invention. As partof the signal generation location there is a transducer system 10 whichincludes a bridge 11 and associated circuitry 12. The bridge 11 is aWheatstone bridge, which basically is implemented and fabricated by theuse of piezoresistive SOI pressure transducers. The bridge 11 andassociated circuitry 12 are located at the signal generation location.This location may be the bottom of a drilled shaft for oil explorationor for other purposes. Pressure sensors suitable for use in bridge 11are well known, for example may be those and are described in the abovenoted patents and applications. The output of the bridge or pressuretransducer can be compensated using its inherent resistance versustemperature characteristics to provide a stable ratio metric output overa wide temperature range. Transducers which operate accordingly are alsowell known in the art and are described for example also in the abovenoted patents. Coupled to the output of the transducer is an electronicinterface 12. The electronic interface may be fabricated and implementedby SOI electronic circuits including CMOS transistors.

The entire unit 10 as shown in FIG. 1 which consists of the bridge 11 aswell as electronics 12 may be positioned or inserted into a drilledshaft which would be implemented by techniques as for example used inoil well exploration. This shaft for example may be thousands of meterslong. In any event, the entire signal generation apparatus 10 as shownenclosed in the dashed box is positioned near the bottom end of theshaft. The signal generation apparatus has an output 17 which is coupledto a wire 15. The wire 15 runs from the transducer assembly 10 and iscoupled to output 17 to a measuring site or location including ameasuring circuit 18. The length of the wire 15 may be 10,000 meters orlonger. Also shown is a return conduit 16 which may be a shield for wire15 or may be an actual metal or other conduit used to surround andprotect the wire. As seen at the measuring location which is the otherend of the shaft and can be a field office or other ground location atnormal ambient temperature is the measuring circuit 18. The wire 15 isconnected to a voltage source VMS (voltage at measuring source) via aresistor 19. In the exemplary configuration, shown in FIG. 1 VMS is +5volts while resistor 10 is 100 ohms. The values are by way of exampleonly and other values can be employed. The VMS source supplies operatingpotential (+Vcc) to the transducer system 10 at the signal generationlocation. This VMS source is the sole power source used to bias thebridge 11 as well as to operate the circuitry 12. The resistor 19 is DCconnected to resistor 39 (FIG. 2) associated with MOSFET 38 (FIG. 2). InFIG. 1 resistor 39 is shown, by way of example to be 900 ohms. Thus, asseen in FIG. 1, the structure 10 which consists of the bridge 11 and theelectronics 12 is located in a high temperature environment such as thatfound at the bottom of a shaft or hole drilled for oil or geothermalexploration and to measure pressure. The output 17 of the system 10 isdirected to a single wire 15 which also is associated with a returnshield or conduit 16. The wire 15 can be more than 10,000 meters longand is directed to the monitoring or measuring station whereby theoutput on wire 15 is measured to develop a voltage or an indication atmeasuring circuit 18 indicative of the pressure or other monitoredcondition. While the above noted system shown in FIG. 1 depicts themeasurement of pressure, it is understood that other measurements can bemade such as temperature, etc., utilizing the single wire interface asdescribed.

FIG. 2 shows a block diagram of the one wire interface showing thecircuit details utilized to implement the signal generation system 10 ofFIG. 1. Essentially before proceeding with a detailed discussion of theinterface, a brief description of the operation will be given.

Referring to FIG. 1 the one wire interface transmits the transduceroutput as a frequency modulated signal at output 17. This frequencymodulated signal propagates over wire 15 which also provides the powersupply to the sensor and interface. Also transmitted over the wire 15 isa reference signal which reference signal is processed through the samechain as the transducer output signal. The reference signal is also afrequency modulated signal. The two signals are multiplexed in a timedivision mode, for example one second of the transducer signal followedby one second of the reference signal. The frequencies of the twosignals are measured by the ground equipment as measuring circuit 18 andthen the ratio of the two periods which are the reciprocals of the twofrequencies is calculated. The ratio of the signals is a direct measureof the transducer output, and eliminates all sources of errors. Sucherrors can be significant, due to the high operating temperatures andare caused by multiple sources, as noise source, RF interference and thelike. Therefore, it becomes very difficult to compensate for suchsources. As the transducer output and the reference signal are passedthrough the same chain, and are affected by the same errors, the ratiocalculation eliminates all errors. It does not matter how these errorsare derived or generated, as they will be present both on the transduceroutput signal as well as on the reference signal and therefore can beeliminated by the apparatus and methods depicted herein.

As one will understand, the technique and apparatus can be furtherenhanced such that multiple sensors can be processed through the samechain and the data sent as time multiplexed signals followed by thereference signal. In this way several sensors can be connected to theground measuring system 18 via a single wire 15. It is also understoodthat temperature sensors can also be used with this interface. A RTDelement can be mounted in a bridge configuration using three fixed metalfilm resistors and the output of the bridge multiplexed and sent throughthe same chain as the reference signal. The reference signal should bevery consistent and stable with temperature in order to enhanceoperational effectiveness.

Referring to FIG. 2 the reference signal is derived from the powersupply +Vcc by a resistive divider which is located on the SOI chip.This implementation has shown that the ratio of resistors on the chip isvery stable and consistent from chip to chip. This ratio is determinedby the geometric features of the resistors and stays constant eventhough the value of the individual resistors may change over temperatureand from device to device. Thus the techniques and apparatus describedherein, also eliminates the errors due to variations in the supplyvoltage. Such variations are possible and expected due to the resistanceof extremely long wires. As the transducer output and the resistivedivider output which is the reference signal are proportional to thesupply voltage +Vcc, the ratio calculation eliminates the undesirederror as well. As one will understand the electronic interface as forexample the circuitry 12 of FIG. 1 is integrated on a SOI chip and theoperating temperature of the interface exceeds 250° C., for example. Asone can also understand, by referring to the above noted co-pendingapplication entitled “High Temperature Pressure Sensor System” the SOIcircuitry depicted therein can be employed herein as well, as forexample, FETs, counters, and the like.

Referring to FIG. 2, there is a shown a circuit diagram in block form ofthe electronic interface 10. As seen the transducer 20 is arranged as aWheatstone bridge configuration. The Wheatstone bridge includes fourpiezoresistors such as 21, which are wired in a bridge configuration.The bridge has a ratiometric output which is compensated over thetemperature range. As seen the bridge derives its biasing voltage fromthe voltage source +Vcc which is applied to the bridge via a spanresistor 22. Thus the bridge 20 produces a ratio metric output which iscompensated over the entire temperature range of operation. Such bridgecircuits including those having ratiometric outputs are well known inthe prior art and examples of such bridge circuits employingpiezoresistors are indicated in the above cited patents. As seen theoutput of the bridge is applied to the input terminals of input (IN1) ofa multiplexer 26. Also shown is a resistive divider consisting ofresistors 23, 24 and 25. The resistors 23, 24 and 25 are in series withone terminal of resistor 25 coupled to reference potential or ground andone terminal of resistor 23 coupled to the biasing voltage source +Vcc.

It is noted that the biasing source +Vcc for the resistive divider isthe same biasing source employed for the bridge. The junction betweenresistors 23 and 24 is applied to one input terminal of the multiplexer26 (INφ) while the junction between resistors 24 and 25 is applied tothe other terminal of the multiplexer input (INφ). It is also notedbefore proceeding further that the resistors 23, 24 and 25 are alsodesignated as R1, R2 and R3. The resistors have been so designated astheir values are used in the mathematics which are pertinent to theoperation of the system. The output of the multiplexer 26 (OUT) isapplied to inputs of an instrumentation amplifier 27. The output of theinstrumentation amplifier 27 is applied to the non-inverting input (+)of an operational amplifier 28. The operational amplifier 28 has afeedback resistor 30 also designated as RF which is coupled to a gaincontrol resistor 29 indicated as R gain. The feedback resistor 30 isconnected and at one terminal to the output of operational amplifier 28and at the other terminal to the inverting input (−) of the operationalamplifier 28. Resistor 29 is coupled between the inverting input (−) ofthe operational amplifier 28 and reference potential. The total gain ofthe amplifier arrangement consisting of instrument amplifier 27 and theoperational amplifier 28 is set by the resistor 29. It is of courseunderstood that the instrumentation amplifier 27 which is also anoperational amplifier has a fixed gain such as a gain of 10 for example.The gain of the operational amplifier is controlled by the value ofresistor 29 which is well known.

The output of the amplifier designated as V_(X) is coupled to thenegative terminal of comparator 31. The positive input terminal of thecomparator is driven or coupled to a capacitor 37. The capacitor 37 ischarged by a current source 36 which is positioned in series with thesource electrode of the FET 35, also designated as Q1. As seen thecurrent source 36 is also coupled to the +Vcc supply. The FET 35 alsohas its source electrode coupled to the positive input of comparator 31and of course coupled to the non-ground terminal of capacitor 37. Theoutput of the comparator 31 is coupled to the input of a monostablemultivibrator 33 whose output is coupled to the gate electrode of theFET 35.

The output of the monostable is also coupled to a 4+11 bit counter andlogic circuit 34. The output of the logic circuit 34 is coupled to theselects input (S) of the multiplexer 26. The output of the counter 34 isalso coupled to the gate electrode of the FET 38 having a sourceelectrode coupled to a load resistor 39. The resistor 39 is coupled towire 15 which at the other end has one terminal of resistor 19 coupledat the measuring end. The other terminal of resistor 19 is coupled tothe VMS source. The junction between resistor 19 (FIG. 1) and resistor39 is the +Vcc which is the biasing potential shown in FIG. 2 and usedto bias all circuitry as amplifier 27, 28, comparator 31, mono 33,counter 34 and so on.

Operation of the circuit is as follows. The positive input of thecomparator 31 as indicated is driven by the capacitor 37. The capacitor37 is charged by the current source 36 and discharged by the MOSFETtransistor 35. The output of the comparator when present triggers themonostable circuit 33 when the capacitor 37 voltage reaches thepredetermined value designated as V_(X). As one can see the output ofoperational amplifier 28 is V_(X). When the value of capacitor 37 ischarged to V_(X) the comparator 31 produces an output which triggersmonostable circuit 33. The time period of the monostable multivibrator33 is chosen to be as short as possible but long enough to safelydischarge the capacitor 37. In one particular example, a time durationof one microsecond for the output of the monostable 33 is appropriate.The value of the capacitor 37 and of the current source 36 is chosensuch that they can be easily implemented on a SOI chip. The prescalersize included in module 34 is chosen such that the output pulses willhave a sufficiently low frequency to provide a useful signal afterpassing through the very high capacitance and resistance of the verylong single wire 15 connection to the measuring circuit 18. The outputof the monostable 33 as seen is applied to a 15 bit counter 34 (4+11bits). The counter controls the select pin of the multiplexer 26 andalso controls the gate of the switch MOSFET transistor 38. The firstfour bits of the counter 34 are used as the prescaler and the following11 bits are used as a counter/sequencer.

The monostable output pulses are first divided by 16 and then the outputis applied to the eleven bit sequencer. Thus, as indicated the countercontrols the select pin of the multiplexer 26 as well as the gate of theMOSFET transistor 38. When MOSFET transistor 38 is turned on the loadresistor 39 is inserted in the circuit increasing the currentconsumption of the circuit. The increase in the current consumptionresults in a voltage drop of about one half volt across the 100 ohmresistor shown in FIG. 1 and coupled to the input of the measuringcircuit 18. The eleven bit counter 34 controls the select pin of themultiplexer such that for 1024 periods or the first interval of thesequencer the transducer output is processed by the interface throughthe inputs IN-1 of the multiplexer 26. After this period or interval thereference voltage of 50 millivolts which is derived from the voltagedivider consisting of resistors 23, 24 and 25 is processed throughinputs INφ for another pulse period of 1024 pulses of the prescaler.This is the reference interval. The output of the prescaler drives thegate of the MOSFET transistor 38 generating through the on/off switchingof the load resistor 39, the square wave current pulses, which appear asthe voltage pulses at the input of the measuring system.

Thus, as seen in FIG. 3A, during the first interval A, 1024 measurementpulses are generated producing a first frequency modulated signalindicative of the value of the transducer output signal. During the nextor second interval B, 960 reference pulses are generated producing asecond frequency modulated signal indicative of the reference leveloutput. Thereafter, for a third interval C equivalent to a 64 pulseperiod. Transistor 38 is disabled and no pulses are provided. This 64pulse interval informs the measuring system that the next sequence ofpulses (1024) is the transducer sequence. After counting 1024 pulses the960 reference pulse interval begins and so on. It is also understoodthat the 64 pulse period could be positioned between the 1024 intervaland the 960 interval and serve the same purpose. These intervals areshown in the timing diagram of FIG. 3A, the 64 pulses are intended forallowing the measuring system to discriminate between the transducersignal phase and the reference signal phase. FIG. 3B is an expanded timescale showing transducer pulses T_(P) and the frequency variation aswell as the reference pulses T_(R).

The voltage across capacitor 37 and the monostable output are shown inFIG. 4. Assuming that the capacitor 37 is discharged and a transistor 35is off, the capacitor 37 is then charged linearly by the current source36. The capacitor charges until it reaches the voltage V_(X). At thismoment the comparator 31 output changes state triggering the monostable33 for a short period (e.g., about one microsecond). During this timethe capacitor 37 is fully discharged. Afterwards the monostable 33 turnsoff, the transistor 35 is turned off and the cycle repeats. It isunderstood that the time scale shown in FIGS. 3 and 4 is distorted forclarity purposes. The duration of the monostable pulses and thedischarge time of the capacitor are less than that of a fraction of 1%of the charging time. FIG. 3B shows the pulses as depicted in FIG. 3Aexpanded in time. The pulse edges are not very fast, and in fact areslowed significantly by the very high capacitance of the long wire.Thus, as seen the pressure pulse designated at T_(P) has a relativelyslow rise time and fall time as does the reference pulses designed asT_(R). FIG. 3B as indicated shows an expanded version of the pressuretransducer pulse values as well as the reference pulse values depictedin FIG. 3A.

Referring again to FIG. 4, there is shown the time diagram of thecapacitor 37 voltage in the top diagram and the output of the monostablein the bottom diagram. Thus, as seen when the capacitor voltage reachesV_(X) the monostable multivibrator triggers for a duration of onemicrosecond. After the monostable pulse the cycle repeats again asdepicted in FIG. 4.

Additional pressure transducers can be employed and for any additionaltransducer the multiplexer 26 will need an additional set of inputs andthe counter/sequencer circuit 34 is implemented to provide additionalintervals for the second, third and fourth transducer. As can be seen byreferring to FIG. 3A one can implement multiple cycles concerning acycle A, A₁, A₂ followed by a reference cycle B. In the exemplaryconfiguration, the value of resistor 19 of 100 ohm at the input of themeasuring circuit is arbitrarily chosen. In a preferred embodiment, thevalue would be equal to the characteristic impedance of the wire and theconduit. In this case, the bandwidth of the signal transmission issignificantly higher resulting in a much shorter measurement cycle thanshown allowing multiple transducer data to be sent in a shorter time.Also, the pulses shown in FIGS. 3A and 3B will have much fastertransition times resulting in a better accuracy and noise immunity ofthe period measurement. Another enhancement of the interface can be theaddition of a thermal electric cooler for the electronic chip. This chiphas a very small size and consequently a very small thermal mass. Thus,a small thermal electric cooler could be positioned on the chip tomaintain the chip temperature at safe low levels without significantpower consumption. Such thermal electric coolers also designated asPELTIER coolers are well known and are employed in many electronic chipssuch as microprocessors for use in computers and this will not bedescribed in further detail.

In order to more clearly understand the nature of the invention thefollowing circuit analysis is hereby presented.

Referring again to FIG. 2, in conjunction with FIGS. 3 and 4, assumingthe sensor is a piezoresistive bridge 20 compensated using traditionalways, the bridge output voltage V_(BR) can be written as:V _(BR) =k*p*V _(CC)

where k is the bridge sensitivity, p is the pressure, and V_(CC) is thebridge supply voltage. During the measuring phase, i.e. when IN1 of themultiplexer 26 is selected, the output of the amplifier 28 V_(X) can beexpressed as;V _(X) =G*V _(BR)

where G is the amplifier gain.

The voltage u_(c) across the capacitor 37 is:

$u_{c} = \frac{I_{Q}*t}{C}$where I_(Q) is the capacitor charging current, t is the time and C isthe capacitance 37. I_(Q) is generated by the current source 36. Whenu_(c) reaches the level V_(X) then the comparator 31 changes state, thustriggering the monostable circuit 33 which rapidly discharges thecapacitor 37 through the transistor 35 (Q1). Neglecting the very shortdischarge time, the cycle time T of the capacitor 37 (C) can becalculated by substituting V_(X) instead of u_(c), resulting:

$T = {\frac{C*V_{X}}{I_{Q}}.}$Taking into account the prescaler 34 factor of 16, the period T_(P) ofthe output signal during the measuring phase is:

$T_{P} = {\frac{16*C*V_{X}}{I_{Q}}.}$

By substituting the formulas for V_(X) and V_(BR) the period T_(P)becomes:

$T_{P} = {\frac{16*C*G*k*p*V_{CC}}{I_{Q}} = {k*p*{\frac{16*C*G*V_{CC}}{I_{Q}}.}}}$

Considering now the second phase, when the reference voltage V_(R) isselected by the multiplexer 26 we have:

$V_{R} = {{\frac{R\; 2}{{R\; 1} + {R\; 2} + {R\; 3}}*V_{CC}} = {r*{V_{CC}.}}}$

The factor r is the resistance ratio:

$r = {\frac{R\; 2}{{R\; 1} + {R\; 2} + {R\; 3}}.}$

A similar calculation for the period T_(R) during this reference phaseresults:

$T_{R} = {r*{\frac{16*C*G*V_{CC}}{I_{Q}}.}}$

At this point, it is important to note that the capacitance C 37, thegain G, the supply voltage V_(CC) and the charging current I_(Q) allhave large random shifts with temperature and variations from device todevice, while the factor r is very constant over temperature and fromdevice to device. Calculating the ratio between T_(P) and T_(R) results:

$\frac{T_{P}}{T_{R}} = {\frac{k}{r}*{p.}}$

This allows one to calculate the pressure p as:

$p = {\frac{r}{k}*{\frac{T_{P}}{T_{R}}.}}$This formula shows that the pressure p can be calculated from two timemeasurements of T_(P) and T_(R) and from two very stable and welldefined constants k and r.

For the component values shown on the schematic:

I_(Q)=1 μA

C=100 pF

V_(X)=1V to 4V during the measuring phase

V_(X)=2.5V during the reference phase

resulting in the charging times of the capacitor to be 100 μs to 400 μsduring the measuring phase and 250 μs during the reference phase.

Due to the 4 bit prescaler, i.e. divide by 16, the respective periods inthe transmitted waveform are 16 times longer, i.e. T_(P)=1.6 ms to 6.4ms during the measurement phase, and T_(R)=3.2 ms nominal during thereference phase. The duration of the flat portion of the waveform is 64periods of the reference pulses, corresponding to 204.8 ms.

It is important to note that the same calculations can be done for othermeasurements other than the pressure p. To measure the temperature, abridge with three fixed resistors and an RTD as the fourth arm can beused.

As one can understand the interface as described above has advantagesover the prior art. One main advantage is that there is a reduced numberof wires for multiple transducers resulting in greater cost savings,less complexity, and improved reliability. The electronic circuit isimplemented in a single integrated chip as all the components shown inthe above diagram of FIG. 2 are known building blocks of SOI processingtechniques. Many of the components as seen in the block diagram exhibitlarge changes with temperature when processing a single signal. Theseerrors are cancelled when the ratio of the two periods is calculated.Such errors result, for example, from mismatch of gain resistors,changes in the value of the capacitor (e.g., capacitor 37) changes inthe value of the current source 36, and changes in other circuitcomponents from device to device as well as the effective resistance ofthe long wire and the value of the supply voltage.

As indicated, the circuit can be implemented as a very small integratedcircuit chip with very few external connections. For a singletransducer, the chip requires only five pins, allowing use of a verysmall package. The functions of the external measuring circuit aresimpler than other implementations as the measuring circuit has tomeasure only two time periods which can be done with a comparator anddigital circuits with a much better accuracy and much less complexitythan any other measurement. The circuit can operate from a five voltsupply reducing the power dissipation compared with other circuitsrequiring much higher voltages. There is no need for voltage regulatorsor stable references as all of the pertinent features due to changes andso on are cancelled by performing a ratiometric indication.

It is well known based on the above, as to how the ratiometriccalculation can be performed as there are many circuits well known inthe art which are capable of providing division of, for example T_(P)divided by T_(R) as well as multiplication. All of this can beimplemented by a microprocessor or conventional circuits which arewidely available. As discussed herein, the ratiometric measurementaccording to an aspect of the present invention cancels outsubstantially all variations.

Referring to FIG. 5 there is shown a comparator 52, the positive inputor non-inverting input of comparator 52 is directed to resistor 51 whereone terminal of resistor 51 connected to the +5 volt supply which isequivalent to the VSM supply depicted in FIG. 1. Resistor 51 isequivalent to resistor 19 shown in FIG. 1. The positive input ofcomparator 52 is directed to wire 15 and hence to the output of thesensor interface as depicted in FIG. 2. The inverting input ofcomparator 52 is biased by connecting it to the common terminal of thevoltage divider consisting of resistors 53 and 54. The voltage dividerconsisting of resistors 53 and 54 supplies a voltage of 4.75 or 0.25volts below the supply voltage. This level corresponds to the middle ofthe pulses generated by the sensor interface. The pulses generated bythe sensor interface are shown in FIG. 3B.

The output of the comparator is connected to an input of amicrocontroller or microprocessor 55 which includes a timer 56controlled by a crystal 57. The measuring system as indicated is locatedremotely from the sensor interface and as depicted in FIG. 1 isrepresented as measurement system 18. The measuring system is at thesurface of the ground if the interface 10 is placed in a drilledaperture or a drilled well associated with an oil well, for example. Inany event, the measuring circuit is at a normal temperature such as roomtemperature or ambient temperature. The measuring system determines thetwo periods in the pulses present in one complete cycle and thencalculates the ratio of these periods. This measurement can be done inmany ways, using typical time and frequency measurements. Two suchimplementations are depicted in the figures and are based on identifyingthe start and end of the cycle from the flat portion (for example nopulses, corresponding in duration to the 64 pulse duration) of thewaveform. This is shown in FIG. 3A. The 64 reference level contains nopulses while the period indicative of the pressure transducer outputcontains 1024 measurement pulses, while the duration of the referencesignal is 960 reference pulses. As indicated above, the period of theflat portion of the waveform is 204.8 milliseconds this is much longerthan the periods of the measurement pulses which can vary between 1.6milliseconds to 6.4 milliseconds or the reference pulses approximatelyat a period of 3.2 milliseconds.

Still referring to FIG. 5, the signal from the interface which is thesignal shown in FIG. 3 is applied to the positive input of thecomparator 52, while the negative input of the comparator is biasedbelow the supply voltage at a level corresponding to the middle of thepulses generated by the sensor interface. This can be shown in FIG. 3B.The output of the comparator is applied to the interrupt input (INT) ofa microcontroller or a microprocessor 55. The microcontroller 55 timestamps each interrupt by reading the internal timer 56 as well as themicrocontroller counts these interrupts. By calculating the differencebetween the two successive interrupts, the microcontroller 55 identifiesfirst the flat portion of the waveform, as its duration is much longerthan any pulse in the sequence. After the flat portion is identified,the microcontroller 55 counts 1024 pulses, determines their totalduration and divides the result by 1024, thus determining the period ofthe pulse as corresponding to the sensor data. Immediately after these1024 pulses, the microcontroller counts the next 960 pulses. It thendetermines their total duration and divides the result by 960 thusdetermining the period of the reference pulses.

The ratio of the two periods is then calculated to determine the valueof the quantity to be measured. This, for example, may be pressure inthe case of utilizing a pressure transducer or may be temperature in thecase of using a temperature transducer. Either the pressure or thetemperature transducer is arranged in a bridge circuit, as shown, forexample in FIG. 1. Therefore, one can utilize this technique to measurepressure, temperature or any other value which can be implemented as avoltage at the output of a bridge configuration.

Referring to FIG. 6 there is shown another exemplary method of measuringthe output of the sensor interface. Also seen in FIG. 6 there is aresistor 60 which is also equivalent to resistor 19 of FIG. 1. Theresistor 60 has one terminal coupled to the input of analog-to-digitalconverter 61 and the other terminal coupled to the +5 volt supply. Theinput of the analog-to-digital converter 61 is connected to wire 15 andthus connected to the sensor interface. The output of theanalog-to-digital converter 61 is connected to the input of amicrocontroller 62. The analog-to-digital converter 61 digitizes theincoming waveform. The sampling rate of the analog-to-digital converteris configured to be about 10 times that of the fastest pulses in thesequence resulting in a sampling interval of 160 microseconds. The flatportion of the waveform is easily identified as no major transitionsoccur through relatively long duration of 204.8 milliseconds.

Next the period of a complete cycle is determined as the time betweentwo successive flat portions. The waveform is then digitized for onecomplete cycle and the result stored. The computer then generates datafor a theoretical waveform, with the same structure as the real one.Thus, for example, the computer generates 1024 measurement pulses, 960reference pulses and a flat portion corresponding to 64 referencepulses. The periods of the measurement and the reference pulses arearbitrarily chosen. The cross correlation function of the two waveformsis then calculated while the two periods in the theoretical waveform arevaried until the cross correlation function shows a very short maximumvalue as a peak. The respective period values in the theoreticalwaveform corresponding to this peak represent the actual measurement andthe reference period. Thus, the reference period again is used toproduce the ratio between the pressure level period and the referenceperiod to produce an output indicative of pressure, while undesiredvariations are thereby cancelled.

The programming regarding the microcontroller shown in FIG. 5 and FIG. 6may be understood by one skilled in the art as the steps for producingand implementing the measuring system are clearly described. It isunderstood that there are other techniques which can be employed tomeasure the time period of both the frequency modulated pressuretransducer output and the frequency modulated reference signal leveloutput.

It should be understood by one skilled in the art that there are manyalterations, and variations of the above noted circuitry all of whichare deemed to be encompassed in the spirit and scope of the claimsappended hereto.

1. Apparatus for transmitting a transducer signal to be measured from asignal generation location to a measuring location connected by a singlewire where said transducer signal is subjected to undesired variationscaused by multiple sources, comprising: a transducer positioned at saidsignal generation location and operative when biased by a power sourceto provide an output signal according to a monitored condition, areference level generator coupled to said power source and operative toprovide a reference level output proportional to the value of said powersource, a multiplexer for receiving at one multiplex input saidtransducer output signal and at another input said reference leveloutput to provide at a multiplexer output said transducer signal for afirst interval and said reference level for a second interval, aconverter responsive to said multiplexer output for said transducersignal for converting said transducer signal to a first frequencymodulated signal having a frequency output variation according to thevalue of said transducer output signal during said first interval andfor providing a second frequency modulated signal indicative of saidreference level during said second interval, wherein variations insignals which may be caused by multiple sources are present in bothsignals, and a measuring circuit positioned at said measuring locationand responsive to said modulated signals to provide the ratio of theperiods of said signals wherein said ratio is a direct measure of saidtransducer output signal with said variations substantially eliminated.2. The apparatus according to claim 1, wherein said transducer is apressure transducer having a bridge arrangement formed by pressuresensors.
 3. The apparatus according to claim 2, wherein said bridgearrangement is a Wheatstone bridge and said pressure sensors arepiezoresistive sensors.
 4. The apparatus according to claim 2, whereinsaid reference level generator is a resistor divider coupled betweensaid power source and a point of reference potential and having a tap onsaid divider for providing a reference voltage level proportional to themagnitude of said power source level.
 5. The apparatus according toclaim 4, wherein said resistor divider is located in close proximity tosaid pressure transducer wherein said divider is exposed to relativelythe same ambient temperature as said pressure transducer.
 6. Theapparatus according to claim 1, wherein said converter furthercomprises: an amplifier having a given gain and having an input coupledto said multiplexer output, said amplifier having an output providing anamplified version of said multiplexer output, a switchable comparatorhaving one input coupled to said amplifier output and having anothercomparator input, and having a switching terminal to switch saidcomparator output from one input to said other input, a capacitor havingone terminal coupled to a point of reference potential and the otherterminal coupled to said another comparator input, said comparatorhaving an output for providing a pulse when said capacitor charges to apredetermined level relatively equal to the level of said amplifieroutput, a switchable current source coupled to said power source forproviding a charging current for said capacitor during a first mode whensaid comparator output does not provide said pulse, and for dischargingsaid capacitor during a second mode when said pulse is present, acounter responsive to said comparator pulses to count said pulses duringsaid first interval indicative of said pressure and for counting saidpulses during said second interval indicative of said reference signallevel said counter having an output coupled to said multiplexerswitching terminal for switching said multiplexer to provide saidpressure output during said first interval and to provide said referencelevel during a second interval with the frequency of said pulses varyingduring both intervals according to the pulses provided at saidcomparator output, means coupled to said counter output to provide saidpulses to said wire and therefore to said measuring location.
 7. Theapparatus according to claim 6, wherein said amplifier includes anoperational amplifier having an inverting input coupled to saidmultiplexer output and a non-inverting input coupled to a gain controlresistor, said amplifier having an output with a feedback resistorcoupled between said output and said non-inverting terminal where saidfeedback resistor and said gain control resistor determines the gain ofsaid operational amplifier.
 8. The apparatus according to claim 6,further comprising: a trigger pulse circuit having an input coupled tosaid comparator output for producing a fixed duration pulse at an outputwhen said capacitor level reaches said amplifier output level, a firstFET having a gate, source and drain electrode, with said gate electrodecoupled to said pulse trigger output, with said drain electrode coupledto a point of reference potential and with said source electrode coupledto a current source, wherein when said pulse is provided at said pulsecircuit output said capacitor discharges, said output of said triggerpulse circuit also coupled to said counter input to supply pulses tosaid counter.
 9. The apparatus according to claim 8, further comprisinga second FET having a gate, source and drain electrode with said drainelectrode coupled to a point of reference potential with said gateelectrode coupled said counter output and with said source electrodecoupled to said wire via a resistor.
 10. The apparatus according toclaim 8, wherein said transducer, said level generator, saidmultiplexer, said amplifier, said comparator, said counter, said FETsand said pulse trigger circuit are all positioned on a single SOIcircuit chip.
 11. A method for transmitting a transducer signal to bemeasured from a signal generation location to a measuring locationconnected by a single wire, said transducer signal subjected tovariations caused by multiple sources, comprising the steps of: placinga transducer at said signal generation location, placing a referencesignal generator in close proximity to said transducer, converting saidtransducer output to a first frequency modulated signal, converting saidreference signal output to a second frequency modulated signal,transmitting said first signal along said wire for a first giveninterval, transmitting said second signal along said wire for a secondinterval different than said first, measuring said first and secondsignals at said measuring location to provide the ratio of the periodsof said signals which ratio is indicative of the transducer outputrelatively free of all variations caused by said multiple sources. 12.The method according to claim 11, further comprising the steps of:multiplexing said first and second signals, and controlling saidmultiplexing to transmit said first signal for said first interval andsaid second signal for said second interval.
 13. The method according toclaim 11, wherein the step of placing a transducer includes placing apressure transducer at said signal generation location.
 14. The methodaccording to claim 11, further comprising transmitting a transitionsignal along said wire after said first interval and said secondintervals to indicate a transition.
 15. The method according to claim13, wherein said signal generation location is the bottom end of adrilled oil well shaft and where said measuring location is at the topend of said shaft.
 16. The method according to claim 15, wherein saidpressure transducer is a piezoresistive transducer having piezoresistorsensors arranged in a Wheatstone bridge array.
 17. The method accordingto claim 11, wherein the step of placing a transducer includes placing atemperature transducer.
 18. Apparatus for transmitting a pressuretransducer signal to be measured from a signal generation locationlocated at one end near the bottom of a drilled oil well shaft extendingto a depth of over 1000 meters and associated with temperatures of over150° C., said shaft extending to the surface where a measuring locationis located and where said signal generation location and said measuringlocation are connected by a single wire, said transducer signalundesirably subjected to variations caused by multiple sources,comprising: a pressure transducer positioned at said one end of saidshaft, and operative to provide an output signal when biased by a powersource according to the value of pressure at said one end, a referencelevel generator located in close proximity to said pressure transducerand operative to provide a reference signal level, said reference levelgenerator coupled to said power source to provide a reference signalproportional to said value of said power source, switching means coupledto said pressure transducer and said reference level generator to enablesaid pressure transducer output to be transmitted at an output for afirst interval and to enable said reference signal to be transmitted fora second interval at said output, converting means coupled to saidswitching means output for converting said pressure transducer output toa first frequency modulated signal and to convert said reference signalto a second frequency modulated signal where said undesirable variationsare present in both frequency modulated signals, and measuring meanspositioned at said measuring location and responsive to said first andsecond frequency modulated signals to provide the ratio of the periodsof said signals where said ratio is a direct measure of said pressuretransducer signal with all of said variations substantially eliminatedreference signal output.
 19. The apparatus according to claim 18,wherein said pressure transducer, said reference level generator, saidswitching means and said converting means are all positioned on the samesubstrate.
 20. The apparatus according to claim 19, wherein saidsubstrate is an SOI substrate.